Pathological anatomies such as tumors and lesions can be treated with an invasive procedure, such as surgery, which can be harmful and full of risks for the patient. A non-invasive method to treat a pathological anatomy (e.g., tumor, lesion, vascular malformation, nerve disorder, etc.) is external beam radiation therapy, which typically uses a linear accelerator (LINAC) to generate x-rays. In one type of external beam radiation therapy, an external radiation source directs a sequence of x-ray beams at a tumor site from multiple angles, with the patient positioned so the tumor is at the center of rotation (isocenter) of the beam. As the angle of the radiation source changes, every beam passes through the tumor site, but passes through a different area of healthy tissue on its way to and from the tumor. As a result, the cumulative radiation dose at the tumor is high and the average radiation dose to healthy tissue is low.
The term “radiosurgery” refers to a procedure in which radiation is applied to a target region at levels that are sufficient to necrotize a pathology. Radiosurgery is typically characterized by relatively high radiation doses per treatment (e.g., 1000-2000 centiGray), extended treatment times (e.g., 45-60 minutes per treatment) and hypo-fractionation (e.g., one to three days of treatment). The term “radiotherapy” refers to a procedure in which radiation is applied to a target region for therapeutic, rather than necrotic, purposes. Radiotherapy is typically characterized by a low dose per treatment (e.g., 100-200 centiGray), shorter treatment times (e.g., 10 to 30 minutes per treatment) and hyper-fractionation (e.g., 30 to 45 days of treatment). For convenience, the term “radiation treatment” is used herein to encompass both radiosurgery and radiotherapy unless otherwise noted.
Image-guided radiation treatment (IGRT) systems include gantry-based systems and robot-based systems. In gantry-based systems, the radiation source is attached to a gantry that moves around a center of rotation (isocenter) in a single plane. The radiation source may be rigidly attached to the gantry or attached by a gimbaled mechanism. Each time a radiation beam is delivered during treatment, the axis of the beam passes through the isocenter. Treatment locations are, therefore, limited by the rotation range of the radiation source, the angular range of the gimbaled mechanism and the degrees of freedom of a patient positioning system. In robot-based systems, such as the CYBERKNIFE® system, developed by Accuray Incorporated of Sunnyvale, Calif., the radiation source is not constrained to a single plane of rotation and has five or more degrees of freedom.
In conventional image-guided radiation treatment systems, patient tracking during treatment is accomplished by comparing two-dimensional (2D) in-treatment x-ray images of the patient to 2D digitally reconstructed radiographs (DRRs) derived from three dimensional (3D) pre-treatment diagnostic imaging data of the patient. The pre-treatment imaging data may be computed tomography (CT) data, cone-beam CT, magnetic resonance imaging (MRI) data, positron emission tomography (PET) data or 3D rotational angiography (3DRA), for example. Typically, the in-treatment x-ray imaging system is stereoscopic, producing images of the patient from two or more different points of view (e.g., orthogonal projections).
A DRR is a synthetic x-ray image generated by casting (mathematically projecting) rays through the 3D imaging data, simulating a known geometry of the in-treatment x-ray imaging system. The resulting DRR then has the same scale and point of view as the in-treatment x-ray imaging system, and can be compared with the in-treatment x-ray images to determine the position and orientation of the patient (and the radiation target within the patient). Different patient poses are simulated by performing 3D transformations (rotations and translations) on the 3D imaging data before each DRR is generated.
Each comparison of an in-treatment x-ray image with a DRR produces a similarity measure or, equivalently, a difference measure, which can be used to search for a 3D transformation that produces a DRR with a higher similarity measure to the in-treatment x-ray image. Similarity measures may be intensity-based or feature-based (e.g., using internal or external fiducial markers or natural anatomical features such as the spine or skull). When the similarity measure is sufficiently maximized (or equivalently, a difference measure is minimized), the corresponding 3D transformation can be used to align the patient in the radiation treatment system so that the actual treatment conforms to the treatment plan.
Conventionally, these treatment systems require two stereoscopic in-treatment x-ray images to insure that the patient is properly positioned in the 3D coordinates of the treatment system before the treatment is started, and these images are acquired periodically during the treatment session. As noted above, the positioning of the radiation treatment source follows a plan that is designed to achieve a target radiation dose to the pathological anatomy, while limiting the radiation dose to critical structures and other healthy tissue. If the treatment plan does not account for the geometry of the in-treatment imaging system, the radiation treatment source may block one of the x-ray imaging beam paths and interfere with stereoscopic imaging.
Conventionally, in order to verify the patient position, the gantry or robot, respectively, must be moved to clear the blocked line of sight of the imaging system, and then be moved back to apply the treatment beam. This procedure wastes time and prolongs the patient's time in the operating theater.
In other situations, the radiation treatment source may not be blocking an imaging path, but one of the two stereoscopic images may not be useable for patient tracking. For example, fiducial markers or anatomical landmarks (e.g., bony structures such as the skull or spine) may be visible in only one of the images. In another example, intensity variations in one of the two images may be too low to guarantee a high quality similarity measure for pattern intensity matching.